Gate Hold

As
you are waiting in your seat, ready to go, the Captain announces there
is a gate hold in effect for your destination. What does that mean?

Gate
hold is more of a slang term for the formal FAA procedures issued under
the Airspace Flow Program (AFP), Ground Delay Program (GDP), and/or
Ground Stop (GS). The AFP is for help with enroute traffic management during severe weather. The GDP
is a traffic management procedure where aircraft are delayed at the
departure airport in order to better match the demand with the capacity
at your destination airport. Both the AFP and GDP can result in the
issuance of an EDCT (Expected Departure Clearance Time) for your flight or a "wheels up time". The GS
is used to preclude long amounts of airborne holding. It helps the
different air control sectors from reaching unsafe saturation levels,
and/or airports of reaching gridlock.

These are put into effect by the FAA's Traffic Management (Flow Control), not the individual airline or airport.
A gate hold for a particular destination is usually for bad weather,
which forces aircraft to stack up in a holding pattern, thus the
airspace becomes more congested. Also, bad weather forces some airports
to use only one runway.

This
can be true for airports such as San Francisco International. Aircraft
usually land to the west (runways 28L and 28R) and the standard arrival
rate (95-99 flights per hour) is based on the fact that both runways can
be used. This assumption is based on good weather or the use of visual
flight rules in a general sense. When the weather turns bad, and
visibility is reduced, the arrival rate is essentially halved (67-72
flights per hour) because only one runway can be used for landing. Tied
in with this decision to reduce to the one runway use is the fact that the FAA has a requirement that when the weather is bad enough to have to use instruments instead of visual references, simultaneous parallel ILS (Instrument Landing System - a approach to be discussed in another topic) approaches cannot be conducted if the runway centerlines are closer then 4300 feet. The 28L,28R runways are only 750 feet apart.

Gusty or strong winds can also have an effect on runway use, thus the arrival rate, and another reason for the gate hold.
For example, using SFO again, the arrival and departure flow is based
on aircraft landing to the west and taking off to the north (runways 1L
and 1R). If the winds are very strong on either a steady or gusty basis,
or both; they can exceed certain aircraft performance limitations. Now
instead of taking off on the normal runway (1L or 1R), they have to taxi
down to the 28 runways. As you can see, now taxi time is lengthened but
more importantly, the air traffic folks must now accommodate departure
aircraft on the arrival runways. So for safety margins to be maintained,
the arrival rate now must be slowed. A domino effect that can
ultimately be just another reason for your gate hold.

Many
times you will be talking to a friend or relative at your destination
and they say, "hey its' good weather, the airline or the FAA must be
lying!!!" Well, what you or your friend can't see or know is that there can be delays because of enroute weather that can't be seen by people at your destination.
A good example is flying to any of the New York area airports. Already
congested on a good day because of volume, there can be enroute
thunderstorms building over eastern Pennsylvania and lower New York
state that cuts down on the amount of airspace available to conduct the
needed enroute traffic separations required (once again, for obvious
safety reasons). More aircraft being funneled between thunderstorms and
narrower spaces greatly reduces the flexibility the air traffic folks
have to get you safely to your destination.

Other
reasons for gate holds that we have encountered include power outages
to the airport or air traffic facilities that will be controlling your
flight (radar out, computers down, no communication), volume control,
and runway(s) closed for emergencies.